• CdTe sensor
• Chromatic Photon Counting
• digital radiography
• hybrid pixel detector
• pixelated detector
• X-ray radiography

The purpose of this work is to characterize Pixirad (INFN spin-off) detector system,
an innovative X-ray imaging sensor with intrinsic digital characteristics, based on
Chromatic Photon Counting technology. The system counts individually the incident
X-ray photons and selects them according to their energy to produce two color images
per exposure. The energy selection occurs in real time and at radiographic imaging
speed (GHz global counting rate). Photon counting, color mode and a very fine spatial
resolution allow to obtain a high ratio between image quality and absorbed dose. The
individual building block of the imaging system is a two-side buttable semiconductor
radiation detector made of a thin pixellated CdTe crystal coupled to a Very Large Scale
Integration (VLSI) CMOS pixel ASIC (Application Specific Integrated Circuits).
The first chapter is devoted to introduce the basic processes of interaction of photons
with matter. In the second chapter the most common radiographic metrics are
described with an introduction to the imager systems. The third chapter focuses on
the pixel detectors, describing the physical mechanism of charge collection involved
in the CdTe sensor and explaining the principal components of the front-end electronic
chain.
In the fourth chapter the general features of Pixirad detector system are described.
The chapter five is focused on the characterization of the Pixirad-1 detector system
from the spectroscopic point of view: an energy calibration and resolution has been
carried out using different X-ray sources and the charge sharing fraction has been evaluated by exploiting the monochromatic Elettra synchrotron beam. The aim of chapter
six is the characterization of the imaging performance of the detector with monochromatic
synchrotron radiation at different energies, investigating the linearity of the system,
the ratio between detector counts and number of impinging photons, as a function
of energy and discriminator thresholds, the pixel noise and the spatial resolution. In
the chapter seven are showed some last measurements with PIXIE III: the third and
last generation of Pixirad ASIC. In particular the full recovering of the charge sharing
effect is demonstrated at 62 micron pixel pitch allowing the sharp spectral separation
between the color images.